Role of Endothelium and Nitric Oxide in the Response of Equine Colonic Arterial Rings to Vasoconstrictor Agents

Objective- To determine the in vitro contractile responses of equine colonic arteries to angiotensin II, histamine, serotonin, norepinephrine, prostaglandin F_2α, vasopressin, and a thromboxane-B₂-analogue. Study Design- The tension generated in colonic arterial rings placed in organ baths with oxygenated Tyrode's solution at 37°C after exposure to the previously mentioned chemical agents was measured using force-transducers interfaced with a polygraph. Sample Population- Large colon arterial rings collected from eight horses. Methods- The rings were allowed to equilibrate for 45 minutes after applying 2 g tension. Bath solution was replaced and tension reapplied at 15-minute intervals. Cumulative-concentration-responses were determined for concentrations ranging from 10^-8M to 10^-4M on three vessel groups namely endothelium intact, endothelium denuded, and L-NAME treated. The maximal response for each vessel was considered as 100%; responses to lower concentrations were calculated as a percentage of the maximum. The EC₅₀ value was determined for each concentration-response relationship of each agent. Results- Vessels with denuded endothelium or those incubated with L-NAME had greater contractile responses. Angiotensin, histamine, serotonin, and norepinephrine produced greater maximal responses than the other agents. Endothelium denuded rings had lower EC₅₀ values. Responses to norepinephrine and serotonin were affected less by denudation. Conclusion- Endothelium plays an important role in modulating responses of colonic arterial rings to contractile agents. Endothelium-derived vasodilators, other than nitric oxide, may modulate contractile responses of equine colonic arteries. Clinical Relevance- Endothelial damage associated with colonic vovulus may be a major factor for sustained reduced perfusion after surgical correction.